1. Field of the Invention
The present invention relates to a toner for use in an image formation method, using electrophotography, electrostatic recording, electrostatic printing or the like, in which a toner recycle system is adopted. The present invention also relates to a binder resin for use in the above toner and to an image formation method using the toner, in which a toner recycle system is adopted.
2. Discussion of Background
Generally, in electrophotography, a latent electrostatic image is formed on a latent electrostatic image bearing member provided with a photoconductive layer comprising a photoconductive material through the processes of electrostatic charging and exposure. The latent electrostatic image bearing member is also referred to as "photoconductor".
The latent electrostatic image is developed to a toner image with toner composed of colored particles. The developed toner image is then usually transferred to an image recording material such as a sheet of paper, and fixed thereto, whereby a copy image is formed.
Conventionally, varieties of image fixing methods for fixing the toner image to the image recording material have been known, and a heat roller image fixing method is particularly widely used, since the heat roller image fixing method is capable of attaining high heat transfer efficiency and performing high speed image fixing.
The toner for use in the heat roller image fixing method is basically required to have the following performances: (1) excellent low-temperature image fixing performance by which image fixing can be securely performed at low temperatures, and (2) excellent anti-hot-offset performance by which a fused toner is made it difficult to be transferred to the heat roller at the time of image fixing.
Further, in order to form a clear copy image, the toner is required to have excellent preservation stability in such a way that the toner can be maintained in the form of powder in a stable manner without aggregation, either while in use or while in store.
Furthermore, in order to form high quality images free of fogging a number of times in a stable manner, the toner is required to have properties of being difficult to be crushed even when mechanical shocks, pressure and the like are applied to the toner in a development unit.
Recently, in order to use the toner economically, attention is paid to an image formation method using a toner recycle system, in which a used toner is recovered in the course of a cleaning process for the photoconductor, returned to the development unit, and reused. However, when a conventional toner is used in the above-mentioned image formation method in which the toner recycle system is adopted, there occur problems such as the reduction in image density, the smearing of the background of copy paper, the fogging of images, and the deposition of the toner on a carrier, as the number of copies made is increased. These problems are caused by the toner being deformed or broken by shearing force applied thereto in the course of the recycling process, whereby the toner is finely divided to form finely-divided toner particles. When this takes place, the toner loses its proper chargeable performance, and the finely-divided toner particles reduces the carrier's charge-imparting performance.
As the toners for use in such a recycle system, there are known several toners in which a cross-linked polyester resin is used as a binder resin as disclosed in Japanese Laid-Open Patent Applications 59-14144, 58-14147, 60-176049, 60-176054, 62-127748 and 62-127749. These toner, however, are so vulnerable to mechanical force that when mechanical force is frequently applied to the toners within the development unit in the course of the recycling process, the toners are broken into finely-divided toner particles. The thus formed finely-divided toner particles contaminate the carrier particles to reduce the charge-imparting performance of the carrier, resultantly causing insufficient charging of the toner, and lowering the developing performance of a developer transporting member and other units which are adversely affected by the insufficiently charged toner.
Such recycle systems have been studied not only with respect to the toner therefor, but also with respect to an image formation method. Recycle systems have been in fact developed and various inventions and improvements have been made in an image formation apparatus to be used with a recycle system as well. For instance, in an electrostatic image formation process, an attempt has been made to sue a residual toner remaining on an image bearing member after toner images have been transferred therefrom to an image receiving member.
Conventionally such a residual toner is collected and filled in recovery bottles which are exclusively used for this purpose and is discarded or processed as an industrial waste. Discarding such an industrial waste will, of course, cause environmental pollution problems and constitute wasting of resources. In order to avoid the discarding of such a used toner, various toner recycle systems have been studied.
Japanese Laid-Open Patent Application 63-246780 discloses a system in which a toner transport passage is provided for transporting a recovered toner from the cleaning unit to the development unit, and the recovered toner is used as part of toner to be supplied to the development unit.
Japanese Laid-Open Patent Application 1-118774 discloses a system of recovering the residual toner after in the development unit, without any cleaning unit being provided.
Japanese Laid-Open Patent Application 6-51672 discloses a system in which a bias-voltage applicable rotary member for recovering toner is provided, and toner is electrostatically recovered when an area corresponding to a sheet-passing portion of an image bearing member passes, and toner is deposited on the image bearing member when an area corresponding to a non-sheet-passing portion of the image bearing member passes.
These systems, however, have their own shortcomings and are not satisfactory for use in practice. In the system disclosed in Japanese Laid-Open Patent Application 63-246780, the toner transport passage such as a pipe is required, and furthermore, toner transport means such as a screw or a belt is indispensable, so that the system itself tends to become oversized an include complicated mechanisms. In the system disclosed in Japanese Laid-Open Patent Application 1-118774, it is difficult to recover the residual toner in the development unit once the toner is deposited as the residual toner on an image bearing member, since the residual toner is apt to be firmly fixed to the image bearing member, so that the background of the image and the image itself tend to be frequently smeared with the toner. Further, this system cannot easily cope with abnormal operations such as paper jamming, and the recovering operation frequently has adverse effects on the processes after the image bearing member is smeared. The shortcomings of these systems are not limited to the above. In any event, the above conventional systems are not yet satisfactory.